Electrical signals can be encoded onto fiber-optic transmissions by electrooptic modulators. These modulators include electro-optic materials having highly polarizable electrons. When these materials are subject to an electric field, their polarization changes dramatically resulting in an increase in the index of refraction of the material and an accompanying decrease in the velocity of light travelling through the material. This electric field-dependent index of refraction can be used to encode electric signals onto optical signals. Uses include, for example, switching optical signals and steering light beams.
A variety of electro-optic materials have been utilized for use in electro-optic devices. Among these materials are inorganic materials such as lithium niobate, semiconductor materials such as gallium arsenide, organic crystalline materials, and electrically-poled polymer films that include organic chromophores. A review of nonlinear optical materials is provided in L. Dalton, “Nonlinear Optical Materials”, Kirk-Othmer Encyclopedia of Chemical Technology, 4th Edition, Volume 17 (John Wiley & Sons, New York, 1995), pp. 288.
In contrast to inorganic materials in which polar optical lattice vibrations diminish their effectiveness, the optical properties of organic nonlinear optical materials depend primarily on the hyperpolarizability of their electrons without a significant adverse contribution from the lattice polarizability. Thus, organic nonlinear optical materials offer advantages for ultrafast electro-optic modulation and switching.
Lithium niobate, a common material currently utilized in electro-optic devices, has an electro-optic coefficient of about 35 pm/V resulting in a typical drive voltage of about 5 volts. Drive voltage (Vπ) refers to the voltage required to produce a π phase shift of light. Lithium niobate has a high dielectric constant (ε=28), which results in a mismatch of electrical and optical waves propagating in the material. The mismatch necessitates a short interaction length, which makes drive voltage reduction through increasing device length unfeasible, thereby limiting the device's bandwidth. Recent lithium niobate modulators have been demonstrated to operate at a bandwidth of over 70 GHz.
Electro-optic poled polymers have also been utilized as modulating materials. Their advantages include their applicability to thin-film waveguiding structures, which are relatively easily fabricated and compatible with existing microelectronic processing. These polymers incorporate organic nonlinear optically active molecules to effect modulation. Because organic materials have low dielectric constants and satisfy the condition that n2=ε, where n is the index of refraction and ε is the dielectric constant, organic electro-optic will have wide bandwidths. The dielectric constant of these materials (ε=2.55-4) relatively closely matches the propagating electrical and optical waves, which provides for a drive voltage in the range of about 1-2 volts and a bandwidth greater than 1 00 GHz.
Advantages of organic nonlinear optical materials include a bandwidth in excess of 100 GHz/cm device and ease of integration with semiconductor devices. See, L. Dalton et al., “Synthesis and Processing of Improved Organic Second-Order Nonlinear Optical Materials for Applications in Photonics”, Chemistry of Materials, Vol. 7, No. 6, pp. 1060-1081 (1995). In contrast to inorganic materials, these organic materials can be systematically modified to improve electro-optic activity by the design and development of new organic materials and by the development of improved processing methods. See, L. Dalton et al., “The Role of London Forces in Defining Noncentrosymmetric Order of High Dipole Moment-High Hyperpolarizability Chromophores in Electrically Poled Polymeric Films”, Proceedings of the National Academy of Sciences USA, Vol. 94, pp. 4842-4847 (1997).
For an organic nonlinear optical material to be suitable for electro-optic applications, the material should have a large molecular optical nonlinearity, referred to as hyperpolarizability (β), and a large dipole moment (μ). A common figure of merit used to compare materials is the value μβ. See Dalton et al. (1997). Organic materials having μβ values greater than about 15,000×10-48 esu that also satisfy the requirements of thermal and chemical stability and low optical loss at operating wavelengths have only recently been prepared. See Dalton et al., “New Class of High Hyperpolarizability Organic Chromophores and Process for Synthesizing the Same”, WO 00/09613. However, materials characterized as having such large μβ values suffer from large intermolecular electrostatic interactions that lead to intermolecular aggregation resulting in light scattering and unacceptably high values of optical loss. See Dalton et al. (1997).
Thus, the effectiveness of organic nonlinear optical materials having high hyperpolarizability and large dipole moments is limited by the tendency of these materials to aggregate when processed into electro-optic devices. The result is a loss of optical nonlinearity. The stability of these materials also limits their utility. Accordingly, there exist a need for improved nonlinear optically active materials having large hyperpolarizabilities, large dipole moments, and high stability and that, when employed in electro-optic devices, are stable and exhibit large electrooptic coefficients. The present invention seeks to fulfill these needs and provides further related advantages.